Biomedical sensing methods employing hydro-insensitive alternating current responsive composites

ABSTRACT

A method is disclosed of detecting a bioelectrical signal from a subject. The method includes the steps of applying a composite material to a subject wherein the composite material includes a polymeric material and a polar material that is substantially dispersed within the polymeric material; coupling monitoring equipment to the second side of the composite material; permitting the polar material within the polymeric material to respond to the bioelectrical signal within the subject; and detecting a responsive electrical signal from the composite material that is representative of the bioelectrical signal. The polar material exhibits molecular compatibility with the polymeric material such that the polar material neither blooms to a surface of the polymeric material nor crystallizes within the polymeric material, and the composite material has a first side for contacting the subject and a second side.

PRIORITY INFORMATION

The present application is a continuation of U.S. patent application Ser. No. 12/751,558 filed Mar. 31, 2010, now U.S. Pat. No. 7,867,611, which is a continuation of U.S. patent application Ser. No. 12/123,773 filed on May 20, 2008, now U.S. Pat. No. 7,713,447, which is a continuation of U.S. patent application Ser. No. 10/410,988 filed on Apr. 10, 2003, now U.S. Pat. No. 7,651,638, which claims priority to U.S. Provisional Patent Application Ser. No. 60/371,306 filed Apr. 10, 2002.

BACKGROUND OF THE INVENTION

The invention relates to polymeric materials that are used to conduct electrical signals, and relates in particular to conductive adhesives that are used with medical monitoring sensors that are placed directly on a patient, such as an electro-cardiogram (EKG) sensor.

Electrically conductive pressure-sensitive adhesives for use in biomedical applications are disclosed, for example, in U.S. Pat. No. 4,848,353. Since such conductive materials typically depend on the presence of water, however, the material must be maintained in a sealed environment until being used. See also, U.S. Pat. No. 5,143,071, which discloses non-stringy adhesive gels for that are hydrophilic.

Such substances must be isolated from the environment prior to use (e.g., in sealed packages), and may function improperly if allowed to lose water from the conductive material. These limitations adversely affect both the cost of sensors that use such conductive adhesives as well as the amount of use that any particular sensor may enjoy.

There is a need therefore, for a material that may be used to conduct electricity yet is not susceptible to variations in the water vapor content of the environment in which it is used.

SUMMARY OF THE INVENTION

In accordance with an embodiment, the invention provides a method of detecting a bioelectrical signal from a subject. The method includes the steps of applying a composite material to a subject wherein the composite material includes a polymeric material and a polar material that is substantially dispersed within the polymeric material; coupling monitoring equipment to the second side of the composite material; permitting the polar material within the polymeric material to respond to the bioelectrical signal within the subject; and detecting a responsive electrical signal from the composite material that is representative of the bioelectrical signal. The polar material exhibits molecular compatibility with the polymeric material such that the polar material neither blooms to a surface of the polymeric material nor crystallizes within the polymeric material, and the composite material has a first side for contacting the subject and a second side.

In accordance with a further embodiment, the invention provides a method of detecting an alternating current bioelectrical signal from a subject, wherein the method includes the steps of applying a non-ionically conductive composite to a subject wherein the non-ionically conductive composite includes a polymeric material and a polar material that is substantially dispersed within the polymeric material, the composite material having a first side for contacting the subject and a second side; coupling monitoring equipment to the second side of the non-ionically conductive composite material; permitting the polar material within the polymeric material to respond to the alternating current bioelectrical signal as the alternating current bioelectric increases within the subject by aligning with the alternating current bioelectrical signal; permitting the polar material within the polymeric material to respond to the alternating current bioelectrical signal as the alternating current bioelectric current signal decreases within the subject by becoming non-aligned with the alternating current bioelectrical signal, thereby providing a discharge electric field as the polar material becomes non-aligned; and detecting the discharge electric field using the monitoring equipment. The discharge electric field is representative of the alternating current bioelectric signal within the subject.

BRIEF DESCRIPTION OF THE DRAWING

The following description may be further understood with reference to the accompanying drawings in which:

FIG. 1 shows an illustrative diagrammatic view of a composite in accordance with an embodiment of the invention;

FIG. 2 shows an illustrative diagrammatic top view of a monitoring sensor using a composite in accordance with an embodiment of the invention;

FIG. 3 shows an illustrative diagrammatic side view of the monitoring sensor shown in FIG. 2 taken along line 3-3 thereof;

FIG. 4A shows an illustrative diagrammatic graphical representation of a patent signal being monitored by a conventional hydro-gel sensor;

FIG. 4B show illustrative diagrammatic graphical representation of a patent signal being monitored by a sensor in accordance with an embodiment of the invention;

FIG. 4C shows an illustrative diagrammatic graphical representation of a patent signal being monitored by a sensor including a polymeric material but no polar material; and

FIG. 5 shows an illustrative diagrammatic top view of a conductor pad using a composite in accordance with an embodiment of the invention; and

FIG. 6 shows an illustrative diagrammatic side view of the conductor pad shown in FIG. 5 taken along line 6-6 thereof.

The drawings are shown for illustrative purposes and are not to scale.

DETAILED DESCRIPTION OF THE INVENTION

It has been discovered that a polar material (such as an organo salt) may be dispersed within a polymeric material of sufficient concentration that the resulting composite may be responsive to the presence of an alternating current yet not be sensitive to water gain or loss from ambient temperature or humidity conditions. For example, as shown in FIG. 1, a composite 10 may include an organo salt 12 that is dispersed within a polymeric material 14. If the organo salt 12 has crystallized out of the polymeric material or has bloomed to the surface, then the salt is not compatible with that given polymer, thus the non-homogenous system will not respond to pick up the alternating current signal. If, on the other hand, the salt becomes dissolved within the polymeric material (rendering the polymeric material clear), then the two materials are compatible, and a substantially homogenous mixture exists to form the desired alternating current responsive blend. When the salt is compatible with the polymer blend, it responds to the rise of an alternating current signal by orienting with the field then returning to a ground state with the collapse of the alternating current field. By responding to the presence of an electric field, therefore, the composite acts as a capacitor in coupling the signal (within a patient) to a sensor.

A suitable combination of polar material and polymeric material may be identified by the following procedure. First, a polar material is combined with the polymeric material in about five different concentrations (typically between about 5% to about 45% by weight). Then the adhesive-salt composite is drawn onto a release liner (of about 1.5 mil), and permitted to dry and cure. The surface of the composite is then inspected after a short period of time. If the salt has crystallized out or bloomed to the surface, then the combination of components is not compatible. If, on the other hand, the composite is clear, it is subject to the next level of compatibility testing. The samples should then be subjected an exposure test in which the samples are exposed to 100° F. with 95% relative humidity for 3 days. The samples are then again inspected to determine whether the polar material has migrated toward either surface. If there has been no migration of the polar material and the composite is clear, then the dielectric constant for the composite is determined and the composite is tested for use as a medical monitoring material. The dielectric constant, for example, may be at least 50 for signals at 200 Hz.

In accordance with an embodiment of the invention, the polymeric material may include an acrylic pressure sensitive adhesive such as the V-95 acrylic pressure sensitive adhesive sold by FLEXcon Corporation of Spencer, Mass. The polar material may include a quaternary ammonium organo salt such as the CHEMAX AS-3106 sold by Chemax Corporation of Beaumont, Tex. The percentage by weight of the polar material in the composite may for example, range from about 5% to about 65%, and preferably ranges from about 10% to about 35%.

As shown in FIGS. 2 and 3, the composite 10 may be used with medical diagnostic sensor pads in which a metallic contact element 16 is surrounded by the composite 10 and coupled to monitoring equipment via a conductor 18. The element 16 and composite 10 are positioned within an adhesive plolyethylene foam carrier 20. In use, the sensor pad is placed on a patient's skin and internal alternating current signals (such as a heart rate) are received by the element 16 through the composite 10 and coupled to the monitoring equipment.

In further embodiments, the composite 10 may be used in a wide variety of other uses, including for example, other diagnostic sensors, as well as applications in fuel cell technology, battery technology, electro luminescence, or any other application in which an electrical current and/or signal is carried through an electrolyte that has at it's source a dependence on water. Composites of the invention are insensitive to the presence or absence of water vapor, and therefore, do not require high levels of maintenance of certain levels of water in packaging and during use. It has been discovered that the shortcomings of having an electrolyte dependent on the presence of water vapor are avoided by using for example organo-salts (quaternary ammonium salts, organo-sulfates, fluoroboarates, and other salt like materials having some organic functionality). The organic functionality facilitates the salt's organic compatibility with the polymeric material. Organic compatability may not be required when a polymer has compatibility with a purely inorganic salt, e.g., Cesium Iodide. An objective is to provide a polar material such as salt that is at least substantially uniformly dispersed throughout the polymeric material. The water insensitive composite may then respond to an ascending/collapsing electric field via a capacitive coupling. In view of the relatively high volume direct current resistance of this type of doped polymer, the sensitivity is towards alternating current (AC) rather than direct current (DC). The composite, therefore, may even be more effective at detecting an AC signal than carrying an electrical current.

As discussed above with reference to FIGS. 2 and 3, composites of the invention may be used in place of electrolytic gels such as hydro-gels in cardiac monitoring pads. The monitoring pad with a composite of the invention described above was used to monitor a person's heart rate and the received signal was compared to signals received using a conventional hydro-gel electrolyte and using the polymeric material without doping with a polar material. As shown in FIG. 4B, the received signal using a composite of the invention (as shown at 24 in FIG. 4A) provided a nearly identical signal as the conventional hydro-gel electrolyte sensor (as shown at 22 in FIG. 4A). FIG. 4C shows that the received signal 26 using the polymeric material alone provided no discernable signal information.

Another variation of these monitoring pads is to provide a polymeric material that has sufficient adhesive qualities to be used itself as the adhesive without requiring the adhesive foam carrier 20. As shown in FIGS. 5 and 6, a conductor pad for use in medical applications may include a composite 30 of the invention that is sufficiently adhesive that no additional adhesive is required to maintain contact between a patient's skin and the pad. In this example, the pad used as a conductor to provide electrical signals via conductor 28 to a metallic conductor element 32. The alternating current field is then coupled to the patient via the composite 30 via the capacitive coupling characteristics of the composite. A supporting material 34 may also be used to provide structural integrity.

Composites of certain embodiments of the invention have also been tested at varying humidity and over extended periods of time and found to exhibit an insignificant amount of change in performance. Such composites have also been found to provide a recovery time after inducing an overloading current of within 5-10 seconds after the overloading condition was ceased. Although both the hydro-gel and composites of the invention act as a parallel capacitor/resistor, the hydro-gel has a DC resistance of about 500 ohms while a doped adhesive of the invention has a DC resistance of about 100K ohms or higher. The hydro-gel has a capacitance in both high (˜10 kHz) and low (˜200 Hz) frequency ranges in the microfarad range. The doped adhesive of an embodiment of the invention has a more pronounced capacitance that varies with frequency effect from about 230 Pico-farads at 10 kHz to about 1 microfarad at 200 Hz. The dielectric constant K must, therefore, be changing with frequency. In fact, at 10 kHz it is discovered that K is about 10, and at 200 Hz, K is about 10,000 or above. When the K is lower the resistance increases, which requires that the pad impedance and matching impedance of the monitoring equipment must be increased.

In other embodiments, the polymeric material may include various polymeric materials such as acrylic adhesives that may be distinguished by the relative composition of their monomers and by their molecular weights. For example, acrylic adhesives such as the DURO-TAK 80-1074 acrylic adhesive, the DURO-TAK 80-136A acrylic adhesive, or the DURO-TAK 87-2852 acrylic adhesive each of which sold by National Starch and Chemical Co. of Bridgewater, N.J. may be used as the polymeric material. In certain embodiments, the polar material may include organo-sulfonium, organic ester salts, organo-metallic materials, organo-borates, phosphates and phosfites etc. In particular, the polar material may include a quaternium 18 & isopropyl alcohol (such as the ARQUAD 2HT-75 product), dicocodimoium chloride & isopropyl alcohol (such as the ARQUAD 2C-75 product), stearyl octyldimonium methosulfate (such as the ARQUAD HTL8-MS product) each of which is sold by Akzo Nobel Surface Chemistry LLC of Chicago, Ill., or PEG-5 cocomonium chloride (such as the ETHOQUAD C/25 product sold by Brenntag N.V. of Deerlijk, Belgium). Suitable further specific combinations of polymeric materials and salts are provided in the following table using the above product names.

Percent Polar Polar Material Polymeric Material Material by Weight ARQUAD 2HT-75 87-2852 20% ARQUAD 2HT-75 87-2852 40% ARQUAD 2C-75 87-2852 20% ARQUAD 2C-75 87-2852 40% ARQUAD 2C-75 80-136A 20% ARQUAD 2C-75 80-136A 40% ARQUAD HTL8-MS 87-2852 20% ARQUAD HTL8-MS 87-2852 40% ARQUAD HTL8-MS 80-136A 20% ARQUAD HTL8-MS 80-136A 40% ETHOQUAD C/25 87-2852 20% ETHOQUAD C/25 87-2852 40% ETHOQUAD C/25 80-136A 20% ETHOQUAD C/25 80-136A 40%

Composites of the invention may be employed in a wide variety of applications involving the coupling of alternating current electrical activity from one location to another, such as other applications involving the monitoring of electrical activity, or the active application of electrical activity, or even the grounding of undesired electrical activity in, for example, conductive housings.

Those skilled in the art will appreciate that numerous modifications and variations may be made to the above disclosed embodiments without departing from the spirit and scope of the invention. 

1. A method of detecting a bioelectrical signal from a subject, said method comprising the steps of: applying a composite material to a subject wherein the composite material includes a polymeric material and a polar material that is substantially dispersed within the polymeric material, wherein said polar material exhibits molecular compatibility with the polymeric material such that the polar material neither blooms to a surface of the polymeric material nor crystallizes within the polymeric material, said composite material having a first side for contacting the subject and a second side; coupling monitoring equipment to the second side of the composite material; permitting the polar material within the polymeric material to respond to the bioelectrical signal within the subject; and detecting a responsive electrical signal from the composite material that is representative of the bioelectrical signal.
 2. The method as claimed in claim 1, wherein said step of permitting the polar material within the polymeric material to respond to the bioelectric signal includes permitting the polar material to become aligned with the bioelectrical signal as the bioelectric signal increases within the subject.
 3. The method as claimed in claim 2, wherein said method further includes the step of permitting the polar material within the polymeric material to respond to the bioelectrical signal as the bioelectric signal decreases within the subject by becoming non-aligned with the bioelectrical signal, thereby providing a discharge electric field as the polar material becomes non-aligned.
 4. The method as claimed in claim 3, wherein said step of detecting a responsive electrical signal from the composite material that is representative of the bioelectrical signal involves detecting the discharge electric field using the monitoring equipment.
 5. The method as claimed in claim 1, wherein said polymeric material is an adhesive.
 6. The method as claimed in claim 1, wherein said polymeric material is an acrylic adhesive.
 7. The method as claimed in claim 1, wherein said polymeric material is a pressure sensitive adhesive.
 8. The method as claimed in claim 1, wherein said polar material is a salt.
 9. The method as claimed in claim 1, wherein said polar material is an organic salt.
 10. The method as claimed in claim 1, wherein said composite includes a concentration of said polar material within said polymeric material of between about 5% and about 65% based on weight.
 11. The method as claimed in claim 1, wherein said composite includes a concentration of said polar material within said polymeric material of between about 10% and about 35% based on weight.
 12. The method as claimed in claim 1, wherein said composite has a DC resistance of at least about 100 k Ohms.
 13. The method as claimed in claim 1, wherein said composite has a dielectric constant that changes in the presence of and changes with alternating electric field.
 14. A method of detecting an alternating current bioelectrical signal from a subject, said method comprising the steps of: applying a non-ionically conductive composite to a subject wherein the non-ionically conductive composite includes a polymeric material and a polar material that is substantially dispersed within the polymeric material, said composite material having a first side for contacting the subject and a second side; coupling monitoring equipment to the second side of the non-ionically conductive composite material; permitting the polar material within the polymeric material to respond to the alternating current bioelectrical signal as the alternating current bioelectric increases within the subject by aligning with the alternating current bioelectrical signal; permitting the polar material within the polymeric material to respond to the alternating current bioelectrical signal as the alternating current bioelectric current signal decreases within the subject by becoming non-aligned with the alternating current bioelectrical signal, thereby providing a discharge electric field as the polar material becomes non-aligned; and detecting the discharge electric field using the monitoring equipment, said discharge electric field being representative of the alternating current bioelectric signal within the subject.
 15. The method as claimed in claim 14, wherein said polymeric material is an acrylic adhesive.
 16. The method as claimed in claim 14, wherein said polymeric material is a pressure sensitive adhesive.
 17. The method as claimed in claim 14, wherein said polar material is an organic salt.
 18. The method as claimed in claim 14, wherein said non-ionically conductive composite includes a concentration of said polar material within said polymeric material of between about 5% and about 65% based on weight.
 19. The method as claimed in claim 14, wherein said non-ionically conductive composite includes a concentration of said polar material within said polymeric material of between about 10% and about 35% based on weight.
 20. The method as claimed in claim 14, wherein said non-ionically conductive composite has a DC resistance of at least about 100 k Ohms.
 21. The method as claimed in claim 14, wherein said non-ionically conductive composite has a dielectric constant that changes in the presence of and changes with alternating electric field. 